Exhaust gas recirculation (EGR) systems divert a portion of the exhaust gases back to the intake to cool combustion temperatures and reduce throttling losses, thus improving vehicle emissions and fuel economy. In turbocharged engines, an EGR system may include a cooled low-pressure EGR (LP-EGR) circuit wherein exhaust gases are diverted after the gases pass through the turbine of the turbocharger and injected before the compressor upon passage through an EGR cooler. Additionally, the EGR system may include a cooled high-pressure EGR (HP-EGR) circuit wherein exhaust gases are diverted before the gases pass through the turbine of the turbocharger and injected downstream of the compressor upon passage through an EGR cooler. The amount of EGR (HP-EGR and/or LP-EGR) routed through the EGR system is measured and adjusted based on engine speed and load during engine operation to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits.
One example EGR system is shown by Styles et al. in US 20120023937. Therein, LP-EGR is provided at a fixed EGR percentage rate of fresh airflow over a large area of an engine map, including from mid-load down to minimum engine load, even as engine load changes. At higher engine loads, the EGR percentage rate is varied based on engine operating conditions. In addition, at very low engine loads and/or engine idle conditions, no EGR (0% EGR) may be delivered. Such an approach improves transient control and extends the use of EGR over a wider range of operating conditions.
However the inventors herein have identified potential issues with EGR systems. As an example, when high rates of EGR are present and low rates of EGR are requested (such as during selected “full” tip-ins), the delay to reach high torque may be unacceptably long. This may be, at least in part, due to the long transport delay in EGR evacuating the intake system as the exhaust gases have to clear from the intake manifold before a full charge of pure air reaches the combustion chamber to produce maximum possible torque. To mitigate the delay in producing the maximum torque, the maximum level of EGR is reduced under steady state conditions, increasing knock, inefficient use of spark retard or combustion mixture enrichment may be required degrading fuel economy, and offsetting the fuel economy benefits of the prior EGR usage.
As another example, when low rates of EGR are present and high rates of EGR are requested (such as during selected partial tip-ins), the delay to reach high EGR dilution may be unacceptably long. This may be, at least in part, due to the long transport delay in EGR filling the intake system as the exhaust gases have to travel though the turbocharger compressor, high-pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. The delay in EGR entering the combustion chamber can also lead to combustion instability, and knock. To mitigate the knock, inefficient use of spark retard or combustion mixture enrichment may be required degrading fuel economy, and offsetting the fuel economy benefits of the prior EGR usage. The abnormal combustion events can also erode drive cycle fuel efficiency and potentially damage the engine.
As a further example, when high rates of EGR are present and low rates of EGR are requested (such as during selected tip-outs), the delay in EGR purging from the air intake system can lead to the presence of residual EGR dilution at low load conditions. The presence of increased intake dilution at low loads can increase combustion instability issues and the propensity for engine misfires. While the flat schedule of Styles may reduce the likelihood of high EGR amounts at lower engine loads, the schedule may also limit the fuel economy benefits of EGR. For example, the flat EGR schedule may result in LP-EGR being provided at some low load points where no fuel economy benefits from EGR are achieved. In some cases, there may even be a fuel penalty associated with the delivery of LP-EGR at the low load point. As another example, the lower EGR at the lower load points may limit the peak EGR rates achievable during subsequent higher load engine operation. The delayed purging of EGR requiring EGR in the engine intake system at low engine loads can also render the intake compressor susceptible to corrosion and condensation. Furthermore, increased condensation may occur at the charge air cooler of a boosted engine system due to the flow of EGR through the cooler. The increased condensation may necessitate additional counter-condensation measures which further reduce engine efficiency and fuel economy.
Some of the above mentioned issues may be addressed by a method for an engine that has an intake plenum that is divided along the entire length from an inlet (coupled to an intake passage) to an outlet coupled to individual cylinder intake ports. One example method comprises: delivering at least intake air into engine cylinders via a first section of a divided intake plenum, delivering at least EGR into engine cylinders via a second, different, section of the divided intake plenum; and adjusting relative flow from each section to the cylinders via valves between the plenum and the cylinders. In this way, engine dilution can be rapidly increased or decreased in the engine to meet the change in EGR demand.
As an example, an engine intake plenum may be divided along the entire length of the plenum, from an inlet (where air is drawn) to an outlet (where flow is delivered to individual cylinders). The plenum may be divided by a divider into a first, upper and a second, lower plenum portion. The lower plenum portion may selectively be coupled to an EGR passage and may be configured to deliver a mixture of air and EGR to the engine cylinders. An amount of EGR in the mixed charge of the lower plenum portion may be controlled by adjusting an opening of an EGR valve coupled in the EGR passage. The upper plenum portion may not be coupled to the EGR passage and thus may be configured to deliver only fresh intake air to the engine cylinders.
During steady-state conditions, a first set of throttle valves coupling the upper plenum portion to the intake port of each engine cylinder may be held closed while a second set of throttle valves coupling the lower plenum portion to the intake port of each engine cylinder may be opened so that a nominal mixture of air and EGR may be delivered to engine cylinders via the lower plenum portion. In response to a decrease in EGR demand to 0% EGR conditions, such as due to a large operator pedal tip-out or an operator pedal tip-in to wide open throttle, a ratio of flow through the plenums may be adjusted to provide the desired dilution as soon as possible. Specifically, the first set of throttle valves coupled to the upper plenum portion may be fully opened while the second set of throttle valves coupled to the lower plenum portion may be fully closed so as to immediately increase the flow of fresh air into the cylinder while also reducing the flow of EGR into the cylinders. The first and second set of throttle valves may be oriented perpendicularly on a commonly actuated shaft such that the opening of one is coordinated with the closing of the other. Alternatively, each set of throttle valves may be independently actuated. By adjusting the valves to adjust the relative flow of fresh air and EGR into the cylinders via distinct portions of a common intake plenum, a faster drop in EGR into the cylinders is enabled than would otherwise have been possible.
In an alternate example, if a rapid change (e.g., decrease) in EGR is demanded while operating in the steady state conditions, such as a change from high EGR conditions to medium EGR conditions, the first set of throttle valves may be partially opened while the second set of throttle valves is partially closed. The EGR valve may then be adjusted based on the EGR demand and the opening of the first and second throttle valves to provide the desired EGR flow into the second lower plenum portion. Once the desired EGR flow is achieved, the first set of throttle valves may be fully closed to disallow further ingestion of fresh air into the cylinders via the upper plenum portion. Concurrently, the second set of throttle valves may be fully opened to allow the desired engine dilution and flow to be delivered to the engine cylinders via the lower plenum portion.
In this way, rapid increases or decreases in EGR demand can be met, reducing issues associated with delays in EGR delivery or purging. By using a divided intake plenum having a distinct portions for delivering fresh air charge and EGR mixed air charge to engine cylinders, engine dilution adjustments can be expedited. By using an intake plenum that is divided along the entire length, the need for distinct intake passages is reduced, providing benefits associated with component reduction. By adjusting the relative flow into the different plenum portions via adjustments to throttle valves, the delivery of EGR and air can be properly coordinated. Overall, dilution adjustments can be expedited, improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.